The present invention relates to novel optical attenuators and optical modulators both using a magneto-optic element that utilizes diffraction phenomena.
The invention relates also to novel magnetic field sensors using a magneto-optic element that utilizes diffraction phenomena.
The invention further concerns a magnetic field (electric current) measuring apparatus and a method suited for the magnetic field (electric current) measurement of power transmission and distribution lines. More particularly, the invention concerns a novel magnetic field measuring apparatus and a method therefor using a sensor unit which is extremely simple in structure and operates independently of service temperature.
An optical attenuator is a device for adjusting the intensity of light incident upon a light-receiving or image pickup element to an optimum value. In optical communications the losses in transmission lines due to splicing, connection, relaying, branching, etc. are much variable, and the intensity of light incident upon the light-receiving unit also varies greatly owing to the losses in optical fibers themselves. To reduce the light intensity to within the dynamic range of the receiver, therefore, optical attenuators are very often employed. Optical attenuators are in frequent use too for the evaluation, calibration, etc. of the performance and reliability of other communication equipment and optical measuring instruments.
Optical attenuators of the prior art use ND filters and are known to be of two types; fixed attenuators which provide a fixed amount of attenuation and variable attenuators which provide variable attenuation. FIG. 2 illustrates a variable attenuator in common use as comprising an attenuator unit 5 of a variable structure and a collimator unit 3. The attenuator unit of the variable attenuator, as shown in FIG. 3, consists of a step-variable attenuation plate 6 and a continuously variable attenuation plate 7. Generally, the former plate includes ND filters arranged turnably for shifting stepwise by 10 dB, and the latter include ND filters so arranged as to change the amount of attenuation continuously from the angle of rotation to cover somewhat broader ranges (e.g., 0-15 dB) than the step width of the former.
The optical attenuators using ND filters, which involve mechanical rotation of the filters for the setting of attenuation amounts, are complex in construction. Moreover, the presence of movable parts in the optical system makes their reliability questionable.
Another type of optical attenuator, a variable attenuator utilizing a magneto-optic effect, is known (Japanese Utility Model Application Public Disclosure No. 63-128522). That attenuator is advantageous in that it has no moving part because the amount of attenuation is adjustable with external magnetic fields. However, it requires polarization prisms to be disposed on both sides of the magneto-optic element so that a particular polarized light component can be taken out to adjust the intensity of light.
Patent Application Public Disclosure No. 2-2508 teaches a variable attenuator that uses a magneto-optic element and depends on diffraction loss for attenuation. The attenuator necessitates no polarization prism and is simple in construction. Its limitation is the inability of continuously changing the attenuation due to on-off control of a solenoid coil.
U.S. Pat. No. 4,148,556 to G. F. Sauter et al. discloses a device capable of modulating the intensity of multimode light beam by taking advantage of the magneto-optic effect. This technique takes out the diffracted light beam into optical fibers and does not utilize only the light beam directed straightly ahead as an output signal.
Magnetic field sensors, using light as a medium, show good insulation and inertness to the influence of electromagnetic induction. These characteristics make them useful in the current measurement for transmission lines and other similar services. In FIG. 17 is shown a typical magnetic field measuring apparatus of the prior art. The optical system of the apparatus is made up of a magneto-optic element 1, optical fibers 4, 4', collimator lenses 3, 3', a polarizer 17, and an analyzer 18. A beam of light emerging from a light source 28 is divided by the collimator lens 3 into parallel rays of light, which in turn are led through the polarizer to form a linearly polarized light. Application of a magnetic field to the magneto-optic element 1 produces a sufficient Faraday effect to cause the plane of polarization of the linearly polarized light to turn in proportion to the strength of the magnetic field. As this light beam passes through the analyzer 18, the quantity of light is changed by the angle of the polarization plane. The light is then collected by the collimator lens 3' into the optical fiber 4' to be detected by a light-receiving element 28.
Here the light beam that has passed the analyzer is divided into two; the light component that oscillates at the magnetic field frequency .omega. and the light component corresponding to the DC component, and the intensity of light I(.omega.) to Io is expressed as EQU I(.omega.)/Io=2V.sub.r LH.omega.
where H.omega. is the AC magnetic field strength of an object being measured that oscillates at the frequency .omega., V.sub.r is the Verdet's constant of the Faraday element, and L is the thickness of the Faraday element. The magnetic field strength H.omega. is simply found from the I(.omega.)/Io by means of a divider 24.
On the other hand, such a magnetic field measuring apparatus has the disadvantage of complex construction of the sensor itself owing to the necessity of disposing the polarizer and analyzer on the opposite sides of the magnegto-optic element. An additional limitation is that, the Verdet's constant V.sub.r of the magneto-optic element being a function of temperature, the measuring environment must be kept constant.
Yet another magnetic field measuring apparatus known in the art is an optical apparatus for measuring electric current or magnetic field revealed, e.g., by Patent Application Public Disclosure No. 1-223359. This apparatus measures a magnetic field by applying a given bias magnetic field different from the AC magnetic field to be measured to the Faraday element of a sensor head, taking out an angular frequency component (E.omega.) which is the same as that of the AC magnetic field of the light beam that passes through the Faraday rotator and a double angular frequency component (E2.omega.) which is twice as much as that of the AC magnetic field, and finding the value of the double angular frequency component (E2.omega.) relative to the same angular frequency component (E.omega.). This technique is advantageous over the other prior art techniques in that the magnetic field strength can be determined in dependently of the V.sub.r that is a function of temperature.
However, like the other conventional techniques, the magnetic field measuring apparatus of Patent Application Public Disclosure No. 1-223359 too must have a polarizer and an analyzer located on both sides of the magneto-optic element. In addition, it requires means for applying a bias magnetic field, thus making the sensor structure more complex than those of the other techniques.
U.S. Pat. No. 4,554,449 to T. Taniuchi et al., U.S. Pat. No. 4,604,577 to H. Matsumura et al., and U.S. Pat. No. 4,581,579 to K. Nagatsuma et al. all teach magnetic field sensors for optical fibers that make use of the Faraday effect. A drawback common to them is the use of a polarizer and an analyzer as with the above-mentioned apparatuses of the prior art.
It is an object of the present invention to provide a novel optical attenuator capable of easily adjusting the amount of attenuation without the need of moving parts and which can be simplified in overall structure.
Another object of the invention is to provide a novel optical modulator capable of easily adjusting the amount and frequency of modulation without the need of moving parts and which can be simplified in overall structure.
Another object of the invention is to provide a novel magnetic field sensor and magnetic field measuring apparatus of a simplified structure each which requires neither polarizer nor analyzer.
Still another object of the invention is to provide a novel magnetic field measuring method which requires neither polarizer nor analyzer.
A further object of the invention is to provide an apparatus and a method for measuring the magnetic fields of transmission lines, etc. without the need of a polarizer, analyzer, or bias-applying means.
The optical attenuators and modulators of the present invention have now been perfected on the basis of the discovery that the use of a magneto-optic material of perpendicular magnetization in a variable applied magnetic field makes it possible to adjust continuously the quantity of light that passes through the material by dint of diffraction phenomena. It has also been found that incorporation of a reflector in the optical system allows the optical attenuators and modulators of the invention to achieve further improvements in the amount of optical attenuation. Further, the sensors and the apparatus and method for magnetic field measurement capable of measuring external magnetic fields without the need of a polarizer or analyzer but by the passage of a light beam through the above-mentioned magneto-optic material of perpendicular magnetization.